Protection of cupriferous surfaces



Patented July 13, 1948 2,445,155 PROTECTION OF CUPRIFEROUS SURFACES Albert J. Saukaitis, Wayne, Pa., assignor to American Chemical Paint Company,

Ambler,

Ifa., a corporation of Delaware ApplicationOctober3, 1944, .Serial No. 557,015

3-Claims. (Cl. 148-.620)

This invention is concerned with retarding or preventing the attack of corrosive influences ,on copper and copperalloy metal surfaces. It is useful .not only where the surface is that of an article compo'sedentirely of copper orof copper alloy, but also and especially where the surface is a layer or coating of cupriferous metal over a sub-surface of iron orsteel. The invention'contemplates treatment of .such surfaces with aqueous solutions containing chromic acidand zinc, as more fully described hereinafter.

The effect of the treatment, as applied to articles of massive metal, is to delay or prevent tarnishing or stainingof thesurface treated. The same effect is produced when the surface treated is a layer of .cupriferous metalover a substratum or base of ironorsteel, buthere an additionaladvantage is secured, .since the treatmentalso reduces or eliminates the ,efiect of deleterious infiuences acting at the interface between the base metal and the coating. Even where a thin lamina of some such protective -metal-as silver, tin, lead or antimony has been appliedin addition to the cupriferouscoating, use of theinvention results in an improved resistance to-corrosiveinfiuences.

The term cupriferousis here used ini-ts custernary technical connotation, as referring to substantially pure copper or toallo'ys in which copper predominateasuch as brass, berylliumcopper, bronze, and-the-like. Similarly, theterm ferriferous refers to iron, or to steel and the ferro alloys generally.

Where the substratum is of iron or steel, it makes little difierenca so far as the present invention is concerned, bywhat method the copper or copper-alloy surface coat was applied. Thus, for example, the process is equally effective whether the coat has been electro-deposited or whether it has been applied by a .simpleimmersion treatment, such as .is described in U. S. Patent 2,217,921, or'by othermeans.

Although the process of theinvention isefiective as'a finaltreatmentfor cupriferous-surfaces, it has additional advantages as an intermediate treatment, preceding the application of an :organic film, such-as paint, varnish, or lacquer. When used as anintermediate treatment, the.pro-

cess-not only increases the. corrosion resistance. of.

the cupri-ferous coat;: it. also improves the ad- '2 hesion of the organic film, prolongs its life,.and enhances its protective effect.

A particularly useful application of the invention is in preventing corrosion where coatings-of copper have been applied to steel which is to be shaped by some such further treatment as drawing or stamping. For examplasteel ammunition components, which have been coated .with copper (as by the process of U. S. Patent 2,217,921, fcrinstance) and which are to be further shaped by drawing, are susceptible tothe development of corrosion both-above and beneath the copper film. Where corrosiondevelops'during storage or shipping, the'copper may not adhere properly to the metal during the drawing operation. If, however, such articles aretreated' in accordance with the present process, corrosion of the type referred tois largely or entirelyprevented.

The invention comprises the treatment of cupriferous surfaces with an aqueous solutionlof zinc chromate. This treatment may be carried out in various ways, depending on the condition of the cupriferous surface or sheath and the 'ultimate appearance desiredin the finishedarticle.

First If the surfaces to be protected are rough, porous, or rather heavily contaminated with adsorbed 0r superficial impurities,.it is desirable to treat them with a relativel strong solution of zinc chromate, containing total chromic acid (CrOs) between 8.5 grams .per liter' and :17-5 grams per liter. Within thelimits stated, it has been found that solutions containing from '15 grams per liter to grams per liter of chromic acid are particularly effective.

Second: If the surfaces are dense, that-ianot porous, and in a smoother polished-condition, for example, or are substantially clean in a chemical sense, a weaker solution may be employed, ranging in concentration from 0,075 gram per liter to 15 grams perliter, calculated astotal dissolved chromic acid CrOs). This more dilute solution is most satisfactory in a range between 0.8 and 2.5 gramsper liter of C1O3.

Third: Under average conditions, improved results may be secured by utilizing the strong solution first and following this treatment with an application of the weaker solution, which then serves also asa rinse.

In any case, the treatment should leaveia residue or film upon the surface treated. In order to ensure that this will occur it is important that a solution should be employed which is at least saturated with respect to the zinc component. In practice, this is ensured by using, in the preparation of the solution, ordinary commercial flake chromic acid and some oxygencontaining zinc compound, such as zinc oxide, zinc hydroxide or zinc carbonate, in quantities sufficient to produce a solution which is approximately saturated with respect to the zinc compound, and then adding zinc carbonate until a precipitate is formed.

If only one solution is used, it is essential that it be saturated as respects the zinc chromate phase in equilibrium therewith. Where a double-dip or two-stage treatment is used, and the stronger solution is saturated with respect to the zinc chromate phase, the second, or weaker solution need not be saturated, although preferably both solutions should have this characteristic. Where the stronger solution is not saturated as respects the zinc chromate phase which is in equilibrium therewith, it is essential that the weaker solution should be saturated in this respect, and under these circumstances especially, a water rinse should be interposed between the first and. second treatments. Otherwise the drag-out from the first bath would introduce unsaturated solution into the second bath, upsetting the chemical balance of the latter.

The solution may be used as a wash or a spray, but is preferably used in the form of a dip bath in which the articles are immersed. The time of treatment will depend chiefly on the concentration of the solution and the temperature at which it is used. For best results. it is desirable to use the solution at a fairly high temperaturesay between 190 F. and its boiling point. In any case, the time involved will be very short. It is important to allow enough time to make certain that the solution thoroughly wets the surface, driving off any air which may happen to be on the surface, and displacing impurities. This may require only a minute-or even less. But where the surface is rough or porous, sometimes as much as five or even ten minutes should be allowed.

After the treatment has been completed, the article is removed from the bath and the solution remaining on the surface is allowed to dry in situ. The hotter the solution, the faster the treated surfaces will dry after being removed from the bath. Heat absorbed by the metal while in the bath will also promote evaporation of moisture after rem-oval from the bath. Heat from external sources, at temperatures up to 300 F., may also be applied, if desired, to facilitate drying. It is an advantage of this invention that high temperatures may be used without impairing the desired result.

The treated and dried surface will be found to have acquired a greatly increased resistance to corrosion, and to be suitable, as is, for many uses. For other uses. it is desirable to apply an organic surface coating-paint, lacquer r enamel, for example. Where painting is contemplated, it is well not to leave on the dried surface excessive amounts of water soluble matter. In this case it is important that the final rinsing solution, the residues of which are dried upon the surface, contain not over 15 grams per liter of chromic acid and preferably not over 2.5 grams per liter of chromic acid. Otherwise the treated surfaces should be rinsed with plain water and dried before the organic surface coating is applied. When this is properly done, it will be found that the organic finish will form a better bond with the treated surface and will have a longer useful life than if it were applied to a similar surface not treated as herein disclosed.

It will, of course, be apparent that variations in the formulation of the treating agents which I propose to employ can be readily calculated without departing from the spirit of the invention. However, as a guide to those skilled in the art, typical formulae are given below for preparing the treating agent of this invention in four different forms. These are preferred embodiments of my invention insofar as the composition of the treating agents is concerned.

FORMULA No. 1

Solid concentrate Pound Chromic acid (ClOs) 0.70 Zinc oxide 0.30

Total 1.00

This concentrate may be used for making up treating or rinsing baths simply by adding it to water at the temperature of operation and stirring for a short time before use. However, since it is very important that the saturation point, as respects the Zinc component, should be reached in the balanced solution used to form the desired coating, and since careless or unskilled operators might fail to attain this with certainty where the foregoing solid concentrate is employed, it is better, in plant operation, to use a liquid concentrate, as described below.

FORMULA No. 2

Liquid concentrate Chromic acid (commercial flake) lbs 7.123 Zinc oxide lbs 2.910 Zinc carbonate, precipitated lbs 0.083 Water, to make gal 1.000

The water is heated to a fairly high temperature, and the chromic acid is dissolved in it. Then the zinc oxide is stirred in gradually, until it is completely dissolved. This may require several hours. When the temperature of the mixture has fallen to F. or below, the zinc carbonate is stirred in for an hour or so. The solution will contain a small amount of solid phase, and this may be allowed to settle out, storing the clear supernatant material for use. Alternatively, the precipitate may be allowed to remain in the solution, since it does no harm if included.

FORMULA No. 3

Strong treating solution Concentrate of Formula No. 2 gal 0.025 Water gal 0.975 Zinc carbonate oz 0.200

The concentrate is diluted with the water, and the solution is heated to the temperature at which it is intended to be used, preferably a temperature near its boiling point. Then the zinc carbonate may be added, with stirring, to bring the solution into balance. Complete saturation with respect to that zinc chromate which is in equilibrium with, the solution is indicated .by the appearance of some undissolved solid phase. After heating and stirring, and after the appearance of the precipitate, the. solution may be used for producingthe desired coating'onicupriferou's surfaces.

Itishouldbe noted that the solution of Formula No. 3 may also be prepared from the solid concentrate of Formula No. 1, especially if it is to be used as an auxiliary bath in a two-stage processthat is, where'saturationas respects the zinc component is not critical.

FORMULA No; 4

Dilute solution Concentrate of Formula No;2 gal 0.0025 Water gal 0.9975 Zinc carbonate (if desired) oz 0.02

It should be pointed out that, as a rinsingsolution following treatment with. a balanced Formula No. 3 solution, this material behaves quite satisfactorily whether or not it is saturated with respect to the zinc-component. For other-purposes,-and especially if this is the only treating solution used, or if it follows treatment with a solution not in chemical balance, it is essential to use the rinsing solution in saturated form. When this is deemed necessary or desirable, zinc carbonatemay be added until a precipitate appears.

From the foregoing formulae it willbe apparent that treating solutions of various strengths may be prepared, For instance, the solution of Formula No. 2 may be diluted to concentrations of between 0.0088% and 20% by volume. In general, however, dilutions of the solution of Formula No. 2 in a range between about 2% and of concentrate by volume will give quite satisfactory results in reference to the time required on the one hand and economy of material on the other when the strong solution of Formula No. 3 is needed. It is, of course, commercially desirable to use as little of the material as will give a satisfactory film in a reasonable length of time. The strength of the solution can be regulated to accomplish this. Generally speaking, processing times between 1 minute and 5 minutes are recommended. If the processing time required is longer than minutes a more concentrated solution should be used; if it is almost instantaneous, it may be desirable, for reasons of economy, to use a more dilute solution. Where conditions are favorable, the weaker solution may safely be used alone, in which case concentrations between 0.09% and 0.30% give quite satisfactory results.

In general, the concentrate as given in Formula No. 1 and also in Formula No. 2 will, when diluted asindicated, produce solutions which are.

approximately saturated with respect to the zinc chromate solid phase in equilibrium therewith. If no solid phase is visible in the diluted solution when it is at the operating temperature, it is wise to add small portions of zinc oxide, zinc hydroxide or zinc carbonate (preferably the latter) until the solution no longer takes up added material, and a solid residue remains after further heating or stirring.

Practical considerations have indicated that the concentrate, whether solid or liquid, should contain at least 25 parts of zinc oxide or an equivalent quantity of zinc carbonate or zinc hydroxide as against 75 parts of chromic acid, calculated as CIOs. Otherwise, too long a time is necessary to bring the solution to the required saturation point by stirring in the zinc carbonat or oxide compound. Similarly, it is wasteful to use a substantial excess of the zinc compound since this will result merely in the appearance ofa large amountiofsolidprecipitate-in the solution.

0f course, some variation is quite feasible, but for all practical. purposes the amount "of zinc-oxide, which is preferably in the neighborhoodof 25 parts, should not exceed 35 parts in proportion to the :chromic acid concentration, which may vary'inverselyfrom'lfi parts to 65 parts. It'may ice-mentioned that the zinc:- chromate ratio has anv important bearing on the'solubility.characteristics. The-proportions indicated will ensure the presence of sufilcientCros to maintain solubility. On-the other hand, an excess of the acid radical should be avoided, particularly where theprocess is:-applied to surfaces which are ultimately covcred-with an organic paint.

The 1 essential characteristics of the compositions herein disclosed are first that they should beicapable offorming a solution which contains enough dissolved chromic acid to produce the de-, sired effect is not much more than'5 or 10 minutes, and second,that the solutions which theyproduce shallbesaturated with respect-to the solid phase (ZnO),s(CrOa)y-(H2O)z, which is in equilibrium with the solution. (It will be understood that the oxygen, containing zinc component indicated as ZnO may be attributable to zinc oxide itself or to-zinc hydroxide, or zinc carbonate, or to mixtures of the same. The subponents indicate-any values which naturally occur when the solutions are saturated.)

Small amounts of foreign ions apparently do not interfere substantially with the effect of the treatment. However, the only ions which appear to be essential to successful operations are those of zinc and chromic acid.

In the accompanying drawing, concentration of chromic acid in grams per liter is plotted against pH values in a, distilled water solution. It will be seen that considerable variation in concentration is possible within the scope of this invention, and that pH values for solutions which are saturated as respects the phase may range from 4.5 to 6.5. The lower the amount of CrOa found in the solution, the higher will be the pH. If tap water is used in preparing solutions which are near the lower limit of concentration herein considered, the eifect of normal impurities on the pH value should be taken into account.

It should be reiterated that whatever the pH value of the solution, it is essential that the primary treating solution, Whether it be the strong solution of Formula No. 3 or the dilute solution of Formula No. 4, should be at least saturated insofar as equilibrium with the zinc component is concerned. Only a smal drop from the saturation point will involve a very substantial drop in the efficiency of the process.

The result of treatment with the zinc chromate solution herein disclosed is the production of a layer of microscopic thickness practically invisible on the surface of copper or copper-alloy materials which have been treated. Yet this thin, transparent film is highly effective. Although an increase of, say, fifty percent in the corrosion resistance of a given article is ordinarily considered to be very significant, yet with the present process there is secured an actual multiplication of the corrosion resistance factor of treated surfaces. Accelerated corrosion tests have repeatedly shown that the process of this invention increases the corrosion resistance of cupriferous surfaces by several hundred percent: in fact the treated surface of several specimen panels has withstood corrosive attack for more than ten timesas long as the untreated surface of the same panels.

I claim: a

1. A process for increasing the resistance of cupriferous surfaces to corrosive influences, which process comprises the steps of treating a cupriferous surface with an aqueous zinc chromate solution, rinsing the surface with water, and treating it with a dilute zinc chromate solution, the solutions being formed by dissolving in water chromic acid and a zinciferolls component consisting of at least one of the group zinc Oxide, zinc hydroxide, and zinc carbonate; the proportion of total dissolved chromic acid in the solution first mentioned being from 8.5 to 175 grams per liter, and the proportion of total dissolved chromic acidin the solution last mentioned being from 0.075 to 15 grams per liter, and at least one of the solutions being saturated as respects the zinc chromate phase in equilibrium therewith; said process further being characterized in that the cupriferous surface is dried by evaporation following treatment with a saturated solution.

2. The process of claim 1 in which the solutions are used at temperatures above about 190 F. but below the boiling point.

3. The process of claim 1, and comprising the further step of applying an organic surface coating to the treated and dried cupriferous surface.

ALBERT J. SAUKAI'IIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Mellor, Inorganic and Theoretical Chemistry, vol. 11 (1931), pages 277-279.

Seidell, Solubilities of Inorganic and Metal Organic Compounds, third ed. (1940), vol. I, page 1584, Van Nostrand, New York. 

